1. Field of the Invention
The present invention pertains generally to semiconductor wafers and in particular to a method for producing and bonding optically flat silicon wafers.
2. Description of the Related Art
Semiconductor material such as silicon is used in several types of optical devices. Silicon, whether in wafer or chip form, is utilized most frequently for its electrical properties. In some devices, however, both the electrical and optical properties of the silicon material are important. An example of such a device is a silicon liquid crystal light valve, such as the one described in U.S. Pat. No. 3,824,002 to Terry D. Beard, entitled "Alternating Current Liquid Crystal Light Valve" and assigned to the same assignee as the present invention. Such a device utilizes a very thin silicon wafer, about five mils thick and about two inches in diameter. It is important that a silicon wafer in a liquid crystal light valve have good optical flatness such as one quarter wavelength or better. It has been found that because the wafer is very thin and somewhat flexible and because of the limitations in polishing techniques, the resulting flatness of the wafers has been less than ideal. Specifically, peak-to-valley deviations are typically on the order of five microns on each side. Because the flatness deviations on one side are independent of those on the other side of the wafer, the thickness variations may be as much as ten microns. In a liquid crystal light valve, it is preferred that the surface of the wafer should be flat to within one micron.
One improved method of producing optically flat silicon wafers is described in U.S. Pat. No. 4,470,856 issued to Little et al. and assigned to the same assignee as the present invention. U.S. Pat. No. 4,470,856 teaches a method for hydrostatically flattening a silicon wafer by pressing the silicon wafer with an optical flat onto a flat baseplate and utilizing a fluid adhesive to secure the wafer to the baseplate. While this method does produce a bonded wafer with acceptable optical flatness, it has some drawbacks. When used in a liquid crystal light valve, the silicon wafer may have a dielectric mirror deposited on its top surface. If the mirror is deposited on the wafer before the hydrostatic flattening operation, contact of the optical flat with the mirror can produce defects in the mirror. If the mirror is deposited after the hydrostatic flattening operation, the fluid adhesive generally cannot withstand the high temperatures necessary for subsequent deposition of the mirror. Also, the layer of glue may distort the resulting image in a liquid crystal light valve if it is not completely uniform in thickness.
In addition, if the wafer has been processed in other ways such as gate oxidation, before bonding to a baseplate, the uneven surface on the wafer caused by such processing will cause the wafer to deform when it is pressed by the optical flat. As a result, any means of attaching the processed wafer to a base plate involving the application of non-uniform pressure is likely to cause unacceptable deformities or defects in the wafer. Thus, it would be desirable to provide a method of producing an optically flat silicon wafer and bonding the wafer without an adhesive or the application of non-uniform pressure or contact with the top surface of the wafer.
Applicant has found that certain advantages result when a technique known as electrostatic bonding is used in the production of optically flat silicon wafers. The technique of electrostatic bonding is described, for example, in U.S. Pat. No. 4,680,243 issued to Shimkunas et al. on Jul. 14, 1987 and the article by P. R. Younger, "Hermetic Glass Sealing By Electrostatic Bonding", Journal of Non-Crystalline Solids, 38 and 39, North-Holland Publishing Company, (1980), 904-914. As discussed in the Younger article, electrostatic bonding is a field assisted sealing technique which requires high temperature to produce ionic conductivity within the glass and high voltage to promote ion migration which allows bond formation to take place. While the exact mechanism of the resulting bond is not well understood, it is believed that an ion exchange occurs during the bonding process. Prior uses of electrostatic bonding have been directed to addressing problems other than the optical flatness of the resulting surface. For example, see U.S. Pat. No. 4,294,602, issued to Horne, which describes a method of electrostatically bonding a borosilicate glass to silicon to protect solar cells from damage due to ultraviolet light. See also the article by M. B. Spitzer et al., "Development of an Electrostatically Bonded Fiber Optic Connection Technique", IEEE Journal of Quantum Electronics, QE18, IEEE (1982), 1584-1588, the article by G. Wallis et al., "Field Assisted Glass-Metal Sealing", Journal of Applied Physics, 10 (1969) 3946-3949, and the article by R. C. Frye, et al., "A field-Assisted Bonding Process for Silicon Dielectric Isolation", J. Electrochem. Soc.: Solid-State Science and Technology, 133 (1986) 1673-1677.
In addition to the above examples, optical flatness is a problem with thin semiconductor wafers even where the device produced only utilizes the electrical properties of the wafer and the final optical characteristics are not critical. For example, in producing many semiconductor devices, precise photolithography techniques are required. Currently, such techniques utilize a vacuum chuck to hold the wafer during the photolithography process. However, it is known that vacuum chucks deform the surface of the wafers in the area where the vacuum is pulling on it. Consequently, during precise photolithography where geometries of three microns or under may be achieved, deviations from flatness caused by the vacuum chuck can cause defects in the devices produced. This is because, for example, when using a production system with a low depth of field, photomasks may not make a good contact over the entire surface of the wafer. Thus, in a four-inch wafer containing a large number of individual circuits, a circuit formed in the area where there is a depression in the wafer will likely be defective. As a result, conventional methods of mounting a thin semiconductor wafer during photolithography limits the size of the individual defect-free circuit which can be produced. This limitation is also a barrier to the goal of achieving wafer scale integration on thin wafers.
Thus, it would be desirable to have a method for temporarily securing a thin flexible semiconductor wafer in a manner which maintains optical flatness during precision photolithography. Such a method would also be useful in any process which requires a semiconductor wafer to be optically flat and which also requires the wafer to be removed without damaging it.